newline in <td title="">
Using 

Works in Chrome to create separate lines in a tooltip.
One way to achieve similar effect would be through CSS:
<td>Cell content.
<div class="popup">
This is the popup.
<br>
Another line of popup.
</div>
</td>
And then use the following in CSS:
td div.popup { display: none; }
td:hover div.popup { display: block; position: absolute; }
You will want to add some borders and background to make the popup look decent, but this should sketch the idea. It has some drawbacks though, for example the popup is not positioned relative to mouse but relative to the containing cell.
This should now work with Internet Explorer, Firefox v12+ and Chrome 28+
<img src="'../images/foo.gif'"
alt="line 1
line 2" title="line 1
line 2">
Try a JavaScript tooltip library for a better result, something like OverLib.
The Extensible Markup Language (XML) 1.1 W3C Recommendation say
« All line breaks MUST have been normalized on input to #xA as described in 2.11 End-of-Line Handling, so the rest of this algorithm operates on text normalized in this way. »
The link is http://www.w3.org/TR/2006/REC-xml11-20060816/#AVNormalize
Then you can write :
<td title="lineone 
 linetwo 
 etc...">